In a traditional display apparatus, a drive circuit for driving an OLED to emit light is a 2T1C (two thin film transistors and one capacitor) circuit which contains only two transistors, wherein the first transistor T1 functions as a switch, and the second transistor DTFT functions as a drive transistor. The operation of the 2T1C circuit is relatively simple. During the operation of the 2T1C circuit, when the scanning signal is at a low level, the first transistor T1 is turned on and the capacitor C is charged by a gray scale voltage on the data line, and when the scanning signal is at a high level, the first transistor T1 is turned off and the gray scale voltage is held in the capacitor C. As the supply voltage is relatively high, the second transistor DTFT is saturated and generates a current for driving the OLED to emit light.
However, there are following technical problems when the traditional 2T1C circuit is used to drive the OLED to emit light: 1) brightness uniformity of the display panel is poor, and brightness of the OLED and brightness of the display panel are lowered; 2) lifetime of the OLED is short.
The technical problem 1) is due to the fact that: a) as the manufacturing process such as Low-Temperature Poly-Si (LTPS) technology is not matured, even if the same technical parameters are used, there are obvious differences among the threshold voltages Vth of the transistors in different positions of a display panel, and as the drive current for driving an OLED to emit light is related to the threshold voltage Vth of the drive transistor, when the same gray scale voltage is inputted, different threshold voltages of the drive transistors will result in different drive currents, resulting in different brightness in different positions of the display panel and poor uniformity of brightness thereof; b) as there is an internal resistance for the circuit, once a current flows through the circuit, a voltage drop must be generated by the internal resistance of the circuit, the voltage difference across the capacitor C will be influenced, for example, the voltage difference across the capacitor C cannot reach a required voltage, thereby brightness of the OLED is lowered; c) with the use of the OLED, many un-recombined carriers are accumulated at the internal interface of the light emitting layer of the OLED, resulting in a built-in electrical field inside the OLED, which causes the threshold voltage Vth of the OLED to drift (in other words, rise steadily), thereby brightness of the OLED is lowered, and brightness of the display panel is lowered.
The technical problem 2) is due to the fact that: with the use of the OLED, some locally conductive microcosmic small channels (filaments) are produced, wherein the filaments are actually caused by some “pinholes” and will influence lifetime of the OLED.
Currently, most of drive circuits for OLED only avoid drift of the threshold voltage of the OLED by using the AC drive to eliminate the locally conductive microcosmic small channels (filaments) of the OLED so that degenerating of the characteristics of the OLED and aging of the OLED are delayed, but influence of the threshold voltage of the drive transistor on brightness of the display panel is not considered; or, most of drive circuits for OLED only compensate for the threshold voltage of the drive transistor to eliminate influence of the threshold voltage of the drive transistor on brightness of the display panel, but degenerating of the characteristics of the OLED and aging of the OLED are not delayed, and lifetime of the OLED is short.